The four components of the blood

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Key points

  • There are four components - or parts - of the blood: red blood cells, white blood cells, plasma and platelets.

  • Blood carries things you need, like oxygen and glucose, and waste products, such as carbon dioxide, around your body.

Component functions

a diagram shoing the parts of the blood. Plasma, red blood cells, platelets, white blood cells
The parts of the blood (not to scale)

Each component has a function.

ComponentFunction
Red blood cellTo carry oxygen from the lungs to every cell in the rest of the body
White blood cellTo fight infection by pathogens and stop disease
PlasmaCarries the blood cells and platelets around the body
PlateletBroken down parts of cells that form scabs

There are different amounts of the four components in the blood.

a test tube showing the parts of the blood and their percentages. Plasma 55%, Platlets and White blood cells 1%, Red blood cells 44%
The components of the blood shown as percentages

Red blood cells

Red blood cells are the most numerous cell in the blood. In one drop of blood there are around 3-5 million red blood cells. They are made in .

Oxygen moves by into red blood cells in the lungs. They have a chemical inside them called haemoglobin, a red pigment which binds with the oxygen to carry oxygen to where it is needed for . This reaction is reversed near the cells.

An image of 3 red blood cells next to the formula haemoglobin plus oxygen equals oxyhaemoglobin

Red blood cells do not have a to maximise the oxygen they can carry. They have dips on both sides to maximise their surface area to absorb oxygen as quickly as possible. This shape is called biconcave.

White blood cells

White blood cells form part of the immune system to keep us safe from infection and disease. There are 10,000-20,000 white blood cells in one drop of blood, and even more when a person is ill. They are made in bone marrow.

There are two types of white blood cell:

  • Phagocytes which surround, engulf and destroy .
  • Lymphocytes which produce antibodies that stick pathogens together. This makes it easier for phagocytes to destroy the pathogens.

Scientists say that phagocytes ‘engulf’ pathogens rather than ‘eat’ them because they do not have a mouth.

Diagram of the 2 types of white blood cell - phagocyte and lymphocyte
The two types of white blood cell. Phagocyte (left) and lymphocyte

Plasma

Plasma is the liquid that makes up more than half of the blood in a human body. It is mainly made of water and is pale yellow so is often called ‘straw-coloured’.

It carries all the platelets and red and white blood cells around the human body. It also carries , dissolved glucose for respiration, dissolved salts and around the body. Waste products like carbon dioxide are also transported in the plasma.

Blood plasma is given to patients who are dehydrated.

Platelets

Platelets are small, colourless fragments of cells that form scabs to stop cuts from bleeding. There are several hundred thousand platelets in each drop of blood.

Like red and white blood cells, platelets are made in bone marrow.

four platelets - small irregular shaped, light coloured parts of the blood
Platelets form scabs to seal the skin and keep infection out

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